Rhodobacter capsulatus xanthine dehydrogenase (XDH) forms an (␣␤) 2 heterotetramer and is highly homologous to homodimeric eukaryotic XDHs. The crystal structures of bovine XDH and R. capsulatus XDH showed that the two proteins have highly similar folds. We have developed an efficient system for the recombinant expression of R. capsulatus XDH in Escherichia coli. The recombinant protein shows spectral features and a range of substrate specificities similar to bovine milk xanthine oxidase. However, R. capsulatus XDH is at least 5 times more active than bovine XDH and, unlike mammalian XDH, does not undergo the conversion to the oxidase form. EPR spectra were obtained for the FeS centers of the enzyme showing an axial signal for FeSI, which is different from that reported for xanthine oxidase. X-ray absorption spectroscopy at the iron and molybdenum K-edge and the tungsten L III -edge have been used to probe the different metal coordinations of variant forms of the enzyme. Based on a mutation identified in a patient suffering from xanthinuria I, the corresponding arginine 135 was substituted to a cysteine in R. capsulatus XDH, and the protein variant was purified and characterized. Two different forms of XDH-R135C were purified, an active (␣␤) 2 heterotetrameric form and an inactive (␣␤) heterodimeric form. The active form contains a full complement of redox centers, whereas in the inactive form the FeSI center is likely to be missing.Xanthine dehydrogenase/oxidase (XDH 1 /XO) is a complex metallo-flavoprotein catalyzing the oxidative hydroxylation of purines, pyrimidines, pterines, and aldehyde substrates using NAD ϩ or molecular oxygen as electron acceptor. Mammalian XDH (EC 1.1.1.204) is a homodimer (290 kDa), with each subunit containing a single molybdenum cofactor (Moco) as well as two iron-sulfur clusters and an FAD molecule. Inherited XDH deficiency, referred to as xanthinuria I, is an autosomal recessive disease that is characterized by hyperuricemia, hyperuricosuria, and xanthinuria and is based on base pair exchanges in the structural gene for XDH (1). The affected individuals may develop urinary tract calculi, acute renal failure, or myositis, because of tissue deposition of xanthine, although some subjects with homozygous xanthinuria remain asymptomatic (1). Several mutations in the human XDH gene in patients with classical xanthinuria have been reported (2-4).In their native form, mammalian xanthine oxidizing enzymes exist as dehydrogenases that transfer electrons to NAD but can undergo a dehydrogenase to oxidase conversion by proteolytic cleavage or by oxidation, with the concomitant loss of the ability to use NAD ϩ as electron acceptor, and nearly 10-fold increase in the oxidase activity. The crystal structures of bovine milk XDH in both the dehydrogenase and oxidase forms have been solved at 2.1 and 2.5 Å resolution, respectively (5). It was shown that during the conversion of XDH to XO, a specific structural rearrangement blocks access of NAD ϩ to its binding site near the FAD moiety and changes ...

The process of globalization is ongoing in the 21st century, with economies, societies and cultures continuing their integration through a globe-spanning network of communication and technology. Under globalization many barriers are removed, although successful globalization retains and defends uniqueness, individuality, and identity. These principles of globalization extend to endoscopy, with the Global Rating Scale (GRS) built around these principles to improve health care. Created in 2004 as a quality improvement and assessment tool for the gastrointestinal endoscopy service it assesses 12 key aspects relating to clinical quality and the quality of the patients′ experience of having an endoscopy (Global Rating Scale, 2008).

Fellowship under the direction of Graham George of the Stanford Synchrotron Radiation Laboratory at the Stanford Linear Accelerator Center. Work supported in part by the Department of Energy contract DE-AC03-76SF00.515.